The present invention relates to a process for decolorization of aqueous solutions of sugars, sugar alcohols and betaine and to the use of polyaluminium chlorides for this purpose.
In this connection, polyaluminium chloride refers to compounds of the general formula EQU Al.sub.2 (OH).sub.x Cl.sub.6-x !.sub.y (I)
wherein 1.ltoreq.x.ltoreq.5 and 1.ltoreq.y.ltoreq.10; and to mixtures of these compounds and corresponding compounds and to mixtures of compounds containing sulphate ions in addition to hydroxyl and chloride ions.
The compounds of the above formula I include, for instance, compounds of the formula EQU Al.sub.n (OH).sub.m Cl.sub.3n-m (II)
which in the case that n=2 has the form EQU Al.sub.2 (OH).sub.m Cl.sub.6-m (III).
In the case that there are also sulphate ions in the product, the compounds can be represented by the general formula EQU Al.sub.2 (OH).sub.n Cl.sub.6-n !.sub.m (SO.sub.4).sub.1 (VI).
Such products, generally known as polyaluminium chlorides, are commercially available; sometimes they are also termed basic aluminium chlorides. They are referred to with the abbreviation PAC, or also with the abbreviation PACS if the product also contains sulphate ions.
A process for preparing compounds included in these products is disclosed in U.S. Pat. No. 3,891,745, in which the products obtained are named basic aluminium chlorides and have the general formula Al.sub.2 (OH).sub.6-n Cl.sub.n, wherein n is a number between 1 and 5.
Polyaluminium chlorides have been used in water purification for coagulation (precipitation) of impurities in waste water. As far as the applicants are aware, however, they have not been used in the decolorization of sugar solutions.
The processes for manufacture of sugar utilized in the sugar industry comprise numerous purification steps of the sugar-containing solution, in which steps the impurities contained in the raw material of sugar are removed in order to obtain pure sucrose. From the crude sugar juice obtained from compression of sugarcane or from the sugar juice obtained from leaching of sugarbeets, pure sugar is manufactured by processes comprising various purification, evaporation and crystallization steps. Part of the coloured impurities is entrapped in the precipitate formed by treatment with lime Ca(OH).sub.2 ! and carbon dioxide; part thereof, however, remains in the solution. In the beet sugar industry, this coloured solution is crystallized and recrystallized (the colured crystals are dissolved and crystallized again), whereby pure white sugar and molasses are finally obtained by means of this recycling, the coloured substances being concentrated in the molasses. In sugar refineries, this solution is decolorized, for instance, by treatment with bone char or activated carbon, and/or by a decolorizing ion exchange, and from the slightly coloured solution thus obtained white sugar can be crystallized with less recrystallization (less recycling). Again, part of the colour is concentrated in the final mother liquid of the crystallization, i.e. in the molasses. Molasses, on the other hand, may be fractionated chromatographically into a sugar fraction and a non-sugar fraction. The sugar fraction produced by this process is also coloured, and the above procedures are again necessary when pure sugar is produced from it.
The solutions of sugar alcohols also often contain such coloured impurities of plant origin or derived from the sugar process, since they are typically produced by the reduction of sugars in the form of a solution. Furthermore, when betaine is produced for example by fractionating molasses, the betaine fraction may contain such coloured substances.
U.S. Pat. No. 4,382,823 and the references cited therein disclose several processes for purification of sugar solutions. Some of them utilize aluminium sulphate in combination with lime, whereby a floc is obtained. This is separated, after a optional addition of a polyelectrolyte, by allowing it to settle over a longer period of time or by flotation, i.e. by aerating the mixture and removing the floc floated in the form of a scum onto the surface of the mixture. Such a flotation process is also the object of U.S. Pat. No. 4,382,823; in this process a floc is formed by means of a combination of lime and either a phosphate ion source or aluminium sulphate, a small part of the mixture to be treated is strongly aerated and combined with the remaining part of the mixture, whereafter polyacrylamide electrolyte solution is added and the resulting floc is allowed to float onto the surface as a scum, which is separated from the clarified sugar solution. Part of the sugar contained in the starting solution is entrapped in the floc and thereby in the scum, from which it can be recovered by dissolving it in water and re-purifying the sugar solution thus obtained by the process described.